Biodiversity gradients in obligate symbiotic organisms: exploring the diversity and traits of lichen propagules across the United States

Abstract Aim Large‐scale distributions of plants and animals have been studied extensively and form the foundation for core concepts and paradigms in biogeography and macroecology. Much less attention has been given to other groups of organisms, particularly obligate symbiotic organisms. We present the first quantitative assessment of how spatial and environmental variables shape the abundance and distribution of obligate symbiotic organisms across nearly an entire subcontinent, using lichen propagules as an example. Location The contiguous United States (excluding Alaska and Hawaii). Methods We use DNA sequence‐based analyses of lichen reproductive propagules from settled dust samples collected from nearly 1300 home exteriors to reconstruct biogeographical correlates of lichen taxonomic and functional diversity. Results Contrary to expectations, we found a weak but significant reverse latitudinal gradient in lichen propagule diversity. Diversity was not impacted by urbanization or human population density. We show that propagules of asexually reproducing species have wider geographical ranges than propagules from sexually reproducing species, likely reflecting the lichenized nature of asexual spores that disperse both the mycobiont and photobiont versus non‐lichenized sexual spores, which disperse only the mycobiont. Main Conclusions Our findings of a reverse latitudinal gradient and a relative lack of impact of urbanization on lichen propagules and/or lichen‐forming fungal spores suggest that core concepts in biogeography are better informed via consideration of additional patterns from other, less well studied groups of organisms.